Viewfinder capable of adjusting diopter

Abstract

An improved eyepiece for a viewfinder of a single lens reflex camera is provided. This improved eyepiece is capable of adjusting both the diopter and astigmatism correction to the demands of the individual user. The eyepiece includes a negative meniscus lens element convex to the object side and a stationary positive lens element. As the negative lens element is movable along the optical axis it adjusts the diopter of the viewfinder. A pair of relatively rotatable prisms is capable of providing variable astigmatism correction.

What is claimed is: 
     
       1. In a viewfinder of a single lens reflex camera having an eyepiece of positive refractive power, the improved eyepiece comprising: a negative lens element movable in the viewfinder along the optical axis thereof; and   a positive lens element stationarily located at the eye side of the negative lens element,   whereby the diopter of the viewfinder is adjustable by means of the axial movement of the negative lens element with the positive lens element left stationary.   
     
     
       2. The invention of claim 1, wherein the refractive power φ 1  of the negative lens element is in the following range:   -0.04<φ.sub.1 <-0.01.     
     
     
       3. The invention of claim 2, wherein the negative lens element is a negative meniscus lens convex to the object side and the eyepiece fulfills the following conditions:   0<1/r.sub.3 ≦0.045       0.9≦r.sub.5 /r.sub.4 ≦1.2     wherein r 3  and r 4  each represent a radius of curvature of the object side and eye side surfaces of the negative lens element, respectively; and r 5  represents a radius of curvature of the object side surface of the positive lens element.   
     
     
       4. The invention of claim 1, wherein the negative lens element is movable within a range less than 5 mm along the optical axis. 
     
     
       5. The invention of claim 1, wherein the overall focal length of the eyepiece is greater than 50 mm. 
     
     
       6. The invention of claim 1 further comprising a detachable member located at the eye side of the positive lens element for compensating abnormal refraction of the eye. 
     
     
       7. The invention of claim 6 wherein the detachable member is a composite lens having a pair of relatively rotatable prism members. 
     
     
       8. In an eyepiece for an optical instrument of a type having a relatively weak refractive power and a relatively limited space for its location, the improvement characterized in that, the eyepiece is divided into an object side negative lens element and an eye side positive lens element, the negative lens element being movable along the optical axis with the positive lens element left stationary, whereby an effective change in diopter is possible by means of the axial movement of the negative lens element within a limited range, in spite of the weak refractive power of the whole eyepiece. 
     
     
       9. The invention of claim 8, wherein the refractive power φ 1  of the negative lens element is in the following range:   -0.04<φ.sub.1 <-0.01.     
     
     
       10. The invention of claim 8, wherein the limited range of axial movement of the negative lens element for obtaining a variable diopter range greater than 2 diopters is less than 5 mm when the overall focal length of the eyepiece is greater than 50 mm. 
     
     
       11. The invention of claim 9 wherein the negative lens elements is a negative meniscus lens convex to the object side and the eyepiece fulfills the following conditions:   0<1/r.sub.3 ≦0.045       0.9≦r.sub.5 /r.sub.4 ≦1.2     wherein r 3  and r 4  each represent a radius of curvature of the object side and eye side surfaces of the negative lens element, respecitvely; and r 5  represents a radius of curvature of the object side surface of the positive lens element.   
     
     
       12. In an improved viewfinder for a single lens reflex camera capable of compensating for individual users variances in diopter and abnormality of eye refraction, the improvement comprising: a negative lens element;   a positive lens element positioned on the eye side of the negative lens element, the negative lens element movable along an optical axis to adjust the diopter of the viewfinder, and   a pair of relatively rotatable prisms capable of providing individual correction of any abnormal refraction of the eye.   
     
     
       13. The invention of claim 12, wherein the negative lens element is a negative meniscus lens convex to the object side and the viewfinder fulfills the following conditions:   -0.04<φ.sub.1 <-0.01       0<1/r.sub.3 =0.045       0.9≦r.sub.5 /r.sub.4 ≦1.2     wherein   r 3  and r 4  each represent a radius of curvature of the object side and eye side surfaces of the negative lens element, respectively;   r 5  represents a radius of curvature of the object side surface of the positive lens elements, and φ 1  is the refractive power of the negative lens element.